The invention relates to a low-pressure mercury vapour discharge lamp comprising a discharge vessel sealed in a gastight manner and filled with mercury and a rare gas. The discharge vessel comprises four parallel extending straight tubular portions positioned in a square. The tubular portions are connected together in a series arrangement to form a discharge passage. Two electrodes are arranged at the ends of two tube parts and are positioned side-by-side on one end of the discharge vessel. A discharge arc is maintained between the two electrodes during operation of the lamp. A lamp of this type is known from U.S. Pat. No. 4,374,340.
Lamps of this type are very compact and if they are provided with an electric stabilisation ballast, a starter and a lamp cap, they are suitable for fitting into holders which are intended for incandescent lamps. The known lamp therefore serves as an alternative to incandescent lamps for general illumination purposes.
Notably lamps provided with an electronic stabilisation ballast and a starter having a relatively small weight (as described, for example, in Netherlands Patent Application No. 8400923 laid open to public inspection) have been found to be very suitable for use in the lamp.
However, it has been found that when the lamp is screwed into the holder, comparatively great forces can be exerted by a user on the tube parts of the discharge vessel. The tube parts are connected together by means of coupling joints located near their ends (as described in the said U.S. Pat. No. 4,374,340) or the tube parts are connected together by means of U-shaped parts (see, for example, DE-OS No. 3,112,878) to which U.S. Pat. No. 4,481,442 corresponds. The coupling joints or U-shaped tube parts are vulnerable and therefore there is a real risk of breakage upon screwing the lamp into a holder.